32 NEW YORK STATE MUSEUM 



tensively developed. Such are the Triassic rocks of the New Jersey 

 side of the Hudson below the Highlands, and the Cretaceous and 

 Tertiary strata of the Atlantic margin on Long Island and Staten 

 Island. The development of underground water supply on Long 

 Island is especially concerned with these later formations, and with 

 the modified drift deposits of the continental margin. 



The whole series of formations are more commonly considered 

 in groups that exhibit certain age or physical unity and that are 

 for the most part characteristic of certain regional belts and that 

 coincide somewhat roughly with the physiographic divisions already 

 noted. There is in the following description and tabulation no direct 

 attempt to unduly emphasize this relation or to belittle the divisions 

 recognized in the commonly adopted geologic column. It is, how- 

 ever, for the purpose in hand, more convenient and useful to keep 

 clear the physical groupings, because largely these groups, instead 

 of the more arbitrary subdivisions of age, are the units used in con- 

 sidering structural and applied problems. 



a Quaternary deposits, (i) Glacial drift. A loose mantle of 

 soil and mixed rock matter covers the bed rock throughout the 

 whole region except (a) here and there where the rock sticks up 

 through (outcrops), and {h) at the most southerly margin along 

 the coast where the glaciers seem not to have reached. 



Origin. This mantle is usually very different in lithologic charac- 

 ter from the underlying rock floor. There is almost always an 

 abrupt break between the rock floor and the overlying material. 

 The rock floor is grooved, smoothed, and scratched as if by the 

 moving of rock or gravel over it. The larger boulders are usually 

 of types of rock identical with ledges lying northward at greater or 

 less distance. Materials of exceedingly great variety both in 

 size and condition and lithologic character are often all piled to- 

 gether in the most hopelessly heterogeneous manner. These are 

 now commonly regarded as conclusive evidence of glacial origin. 

 There is no need of making the discussion exhaustive. It is almost 

 universally called the '' drift.'' 



Thickness. The thickness of the drift varies from almost o to ap- 

 proximately 500 feet. It is generally thickest in the valleys where it 

 has simply filled many of the original depressions and obliterated 

 much of the ruggedness of surface, the gorges and ravines and can- 

 yons of the preglacial time. 



Sources. It appears from an examination of the grooves and 

 striae on bed rock, and the relationship of the different types of 

 drift to each other, and from a comparison of the types of boulders 



